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Abstract:

An interferometry method for determining information about a test object
includes directing test light to the test object positioned at a plane,
wherein one or more properties of the test light vary over a range of
incidence angles at the plane, the properties of the test light being
selected from the group consisting of the spectral content, intensity,
and polarization state; subsequently combining the test light with
reference light to form an interference pattern on a multi-element
detector so that different regions of the detector correspond to
different angles of the test light emerging from the test object, wherein
the test and reference light are derived from a common source; monitoring
the interference pattern using the multi-element detector while varying
an optical path difference between the test light and the reference
light; determining the information about the test object based on the
monitored interference pattern.

Claims:

1. An interferometry method for determining information about a test
object, comprising: directing test light to the test object positioned at
a plane, wherein one or more properties of the test light vary over a
range of incidence angles at the plane, the properties of the test light
being selected from the group consisting of the spectral content,
intensity, and polarization state; subsequently combining the test light
with reference light to form an interference pattern on a multi-element
detector so that different regions of the detector correspond to
different angles of the test light emerging from the test object, wherein
the test and reference light are derived from a common source; monitoring
the interference pattern using the multi-element detector while varying
an optical path difference between the test light and the reference
light; and determining the information about the test object based on the
monitored interference pattern.

2. The method of claim 1, wherein the test object comprises one or more
features and the variation of the one or more properties of the test
light are selected based on the one or more features of the test object.

3. The method of claim 2, wherein the variation of the one or more
properties of the test light are selected so that the information can be
determined with higher sensitivity relative to using test light for which
the one or more properties do not vary across the range of incident
angles.

4. The method of claim 1, further comprising outputting the information
about the test object.

5. The method of claim 1, wherein the information about the test object
comprises information about a refractive index of a layer of the test
object.

6. The method of claim 1, wherein the information about the test object
comprises information about a thickness of a layer of the test object.

7. The method of claim 1, wherein the test object comprises one or more
features and the information about the test object comprises information
about the one or more features.

8. The method of claim 7, wherein the information about the one or more
features comprises a dimension of the one or more features.

9. The method of claim 7, wherein the information about the one or more
features comprises information about a relative position between two or
more of the features.

10. The method of claim 1, further comprising performing a sensitivity
analysis of the information and the one or more properties of the test
light are selected based on the sensitivity analysis.

11. The method of claim 1, wherein directing the test light comprises
modulating the light so that the intensity of the light varies over the
range of incident angles.

12. The method of claim 11, wherein modulating the test light comprises
directing the test light through an aperture corresponding to variation
of the incident angles.

14. The method of claim 11, wherein the test light is modulated using a
spatial light modulator.

15. The method of claim 11, wherein modulating the test light comprises
scanning light into a range of light paths corresponding different angles
within the range of incidence angles.

16. An interferometry method for determining information about a test
object, comprising: directing test light to the test object using a
microscope having an entrance pupil, wherein one or more properties of
the test light vary over the entrance pupil or a surface conjugate to the
entrance pupil, the properties of the test light being selected from the
group consisting of the spectral content, intensity, and polarization
state; subsequently combining the test light with reference light to form
an interference pattern on detector positioned at a surface conjugate to
the entrance pupil of the microscope, wherein the test and reference
light are derived from a common source; monitoring the interference
pattern using the detector while varying an optical path difference
between the test light and the reference light; and determining the
information about the test object based on the monitored interference
pattern.

17. An interferometry method for determining information about a test
object, comprising: directing test light to the test object comprising
one or more features; subsequently combining the test light with
reference light to form an interference pattern on a multi-element
detector so that different regions of the detector correspond to
different angles of the test light emerging from the test object, wherein
the test and reference light are derived from a common source; monitoring
the interference pattern using the multi-element detector while varying
an optical path difference between the test light and the reference
light; determining the information about the test object based on the
monitored interference pattern, wherein directing the test light
comprises selecting a spectral content of the test light based on the
features.

18. An apparatus comprising: a light source module; a scanning
interferometer positioned to receive light from the light source module
and configured to cause test light emerging from a test object positioned
at a plane over a range of angles to interfere with reference light on a
detector so that different regions of the detector correspond to
different angles of the test light emerging from the test object, wherein
the test and reference light are derived from the light source module and
the light source module is configured so that one or more properties of
the test light varies over a range of incidence angles at the plane, the
properties of the test light being selected from the group consisting of
the spectral content, intensity, and polarization state; and an
electronic processing module in communication with the detector, wherein
the apparatus is configured so that during operation the apparatus
monitors the interference pattern at the detector while the scanning
interferometer varies an optical path length between the test and
reference light and the electronic processing module determines
information about the test object based on the monitored interference
pattern.

19. An apparatus comprising: a light source module; a microscope having
an entrance pupil, the microscope being positioned to receive light from
the light source module and configured to cause test light emerging from
a test object to interfere with reference light on a detector, wherein
the test and reference light are derived from the light source module and
the light source module is configured so that one or more properties of
the test light varies over the entrance pupil or a plane conjugate to the
entrance pupil, the properties of the test light being selected from the
group consisting of the spectral content, intensity, and polarization
state; and an electronic processing module in communication with the
detector, wherein the apparatus is configured so that during operation
the apparatus monitors the interference pattern at the detector while the
scanning interferometer varies an optical path length between the test
and reference light and the electronic processing module determines
information about the test object based on the monitored interference
pattern.

20. The apparatus of claim 19, wherein the light source module comprises
one or more light source elements and one or more optical elements
configured to selectively combine light having differing spectral
components from the light source elements.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This claims benefit of Provisional Patent Application No.
61/448,528, filed on Mar. 2, 2011, the entire contents of which are
incorporated herein by reference.

BACKGROUND

[0002] The disclosure relates to optical metrology of surfaces, films, and
unresolved structures, and more particularly to interferometric optical
metrology of surfaces, films, and unresolved structures.

[0003] Interferometric techniques are commonly used to measure the profile
of a surface of an object. To do so, an interferometer combines a
measurement wavefront reflected from the surface of interest with a
reference wavefront reflected from a reference surface to produce an
interferogram. Fringes in the interferogram are indicative of spatial
variations between the surface of interest and the reference surface.

[0004] A scanning interferometer scans the optical path length difference
(OPD) between the reference and measurement legs of the interferometer
over a range comparable to, or larger than, the coherence length of the
interfering wavefronts, to produce a scanning interference signal for
each camera pixel used to measure the interferogram. A limited coherence
length can be produced, for example, by using a white-light source, which
is referred to as scanning white light interferometry (SWLI). A typical
SWLI signal is a few fringes localized near the zero OPD position. The
signal is typically characterized by a sinusoidal carrier modulation (the
"fringes") with bell-shaped fringe-contrast envelope. The conventional
idea underlying SWLI metrology is to make use of the localization of the
fringes to measure surface profiles.

[0005] SWLI processing techniques include two principal trends. The first
approach is to locate the peak or center of the envelope, assuming that
this position corresponds to the zero OPD of a two-beam interferometer
for which one beam reflects from the object surface. The second approach
is to transform the signal into the frequency domain and calculate the
rate of change of phase with wavelength, assuming that an essentially
linear slope is directly proportional to object position. See, e.g., U.S.
Pat. No. 5,398,113 to Peter de Groot. This latter approach is referred to
as Frequency Domain Analysis (FDA).

[0006] Scanning interferometry can be used to measure surface topography
and/or other characteristics of objects having complex surface
structures, such as thin film(s), discrete structures of dissimilar
materials, or discrete structures that are underresolved by the optical
resolution of an interference microscope. By "underresolved" it is meant
that the individual features of the object are not fully separated in a
surface profile image taken using the interference microscope as a
consequence of the limited lateral resolution of the instrument. Surface
topography measurements are relevant to the characterization of flat
panel display components, semiconductor wafer metrology, and in-situ thin
film and dissimilar materials analysis. See, e.g., U.S. Patent
Publication No. US-2004-0189999-A1 to Peter de Groot et al. entitled
"Profiling Complex Surface Structures Using Scanning Interferometry" and
published on Sep. 30, 2004, the contents of which are incorporated herein
by reference, and U.S. Patent Publication No. US-2004-0085544-A1 by Peter
de Groot entitled "Interferometry Method for Ellipsometry, Reflectometry,
and Scatterometry Measurements, Including Characterization of Thin Film
Structures" and published on May 6, 2004, the contents of which are
incorporated herein by reference.

[0007] Other techniques for optically determining information about an
object include ellipsometry and reflectometry. Ellipsometry determines
complex reflectivity of a surface when illuminated at an oblique angle,
e.g., 60°, sometimes with a variable angle or with multiple
wavelengths. To achieve greater resolution than is readily achievable in
a conventional ellipsometer, microellipsometers measure phase and/or
intensity distributions in the back focal plane of the objective, also
known as the pupil plane, where the various illumination angles are
mapped into field positions. Such devices are modernizations of
traditional polarization microscopes or "conoscopes," linked historically
to crystallography and mineralogy, which employs crossed polarizers and a
Bertrand lens to analyze the pupil plane in the presence of birefringent
materials.

[0008] Conventional techniques used for thin film characterization (e.g.,
ellipsometry and reflectometry) rely on the fact that the complex
reflectivity of an unknown optical interface depends both on its
intrinsic characteristics (material properties and thickness of
individual layers) and on three properties of the light that is used for
measuring the reflectivity: wavelength, angle of incidence, and
polarization state. In practice, characterization instruments record
reflectivity fluctuations resulting from varying these parameters over
known ranges. Optimization procedures such as least-squares fits are then
used to get estimates for the unknown parameters by minimizing the
difference between measured reflectivity data and a reflectivity function
derived from a model of the optical structure.

[0009] Pupil Plane Scanning White-Light Interferometry (PUPS) techniques
measure the reflectivity of complex object surfaces (film stacks,
periodic patterns, etc.) as a function of the angle of incidence,
polarization and/or wavelength of the illuminating light. Conventionally,
PUPS measurements involve illuminating the entire pupil of an
interferometer with an extended source having a broad emission spectrum.
The exit pupil of the interferometer is imaged onto a two-dimensional
detector array. The radial position of a detector element defines the
angle of incidence of the light that reflects of the object for that
particular pupil position. The azimuthal position of a detector element
encodes the polarization state of the illumination light in a typical
polarized configuration. The measurement process records an interference
signal at each detector element as the optical path difference between
the sample surface and reference surface of the interferometer is scanned
over some range. Various spectral components of the light source can be
separated by spectral analysis (e.g., Fourier transform) of each
individual interference signal, yielding object complex reflectivity as a
function of angle of incidence, polarization and wavelength.

[0010] Complex reflectivity data generated using PUPS can be compared to
the results of a computation of the reflectivity of a model structure.
The parameters of the models can be optimized iteratively until
experimental and modeled reflectivities are matched. Alternatively the
experimental data can be compared to pre-computed values stored in a
library. The end result can be information about a test object including
information defining a feature of interest in the test object (film
thickness, material optical properties, pitch, CD, depth, undercut,
overlay, etc.).

[0011] Interferometers having multiple modes for determining
characteristics of an object are disclosed in US 2006-0158657 A1 (now
U.S. Pat. No. 7,428,057) and US 2006-0158658 A1, the entire contents both
of which are incorporated herein by reference.

SUMMARY

[0012] A single PUPS measurement can generate many tens of thousands of
independent data points, only a small subset of which may be used to
determine information about the test object. For example, when using PUPS
to characterize 3D unresolved test patterns on semiconductor wafers, the
computation burden imposed by analyzing each data point can be such that
it is not practical to analyze all of the data given the desired
measurement throughput.

[0013] In some cases, a user can perform a sensitivity analysis that
guides the choice of an optimal subset of experimental data points.
However, such analysis reveals that for various features of interest of a
test object (e.g., a film thickness or a grating profile) a PUPS signal
may display a disproportionally higher sensitivity to variations of the
features within a subset of the accessible range of wavelengths, angles
of incidence and/or polarization states.

[0014] If it is given that only a subset of the realizable measurement
points will be used in practice then the overall performance of the tool
(repeatability, accuracy) can be enhanced by optimizing the instrument
configuration and data analysis for this particular subset. The key is to
create an instrument that can be thus optimized for each specific
application with minimum (ideally no) user intervention. The present
disclosure provides a number of enabling hardware and software tools to
achieve such a goal.

[0015] Accordingly, apparatus and methods are presented that feature
illumination profiles having properties (e.g., intensity distributions,
spectral content, and/or polarization distributions) that are tailored to
subsets of the available illumination conditions corresponding to where
PUPS signals are most sensitive to the features of interest of the test
object. For example, in certain aspects, the illumination and
polarization of an interferometer pupil is spatially and/or spectrally
shaped to maximize the signal-to-noise ratio (SNR) and accuracy of the
experimental data. For instance, using a source spectrum composed of
discrete emission lines instead of a continuous spectrum may provide a
number of benefits including: improved SNR of the reflectivity measured
at the discrete frequencies by eliminating the detection noise associated
with other (unused) spectral components; reduced data acquisition time
due to simplified spectral analysis; enhanced accuracy of the measured
reflectivity by elimination of wavelength mixing in the course of the
spectral analysis.

[0016] Such benefits may also be achieved when the full illumination of
the interferometer pupil is converted to a set of discrete points or
lines or circles. For example, in some embodiments, the pupil is
illuminated with discrete rings of light, each ring corresponding to a
specific emission line of the discrete source spectrum. The result is the
collection of multi-wavelength reflectivity information over multiple
angles of incidence while dedicating the dynamic range of single detector
elements to single source frequencies (e.g., optimum SNR).

[0017] In certain embodiments, the pupil is illuminated with discrete
points and the detector is defocused with respect to the image of the
pupil. The amount of defocus can be controlled such that the image of
each discrete illumination point is blurred but does not overlap that of
neighboring illumination points. The sum of the signals recorded by the
detector elements spanning a given blurred spot may provide a signal with
better SNR for the given discrete illumination direction.

[0018] In some embodiments, spectral shaping can include performing a
sequence of measurements using one wavelength (or a narrow spectral
range) at a time while the interferometer setup creates a carrier pattern
in the data recorded by the detector: this enables collecting object
reflectivity at each specific wavelength with a single detector frame,
making the system insensitive to vibration.

[0019] Various aspects of the invention are summarized as follows:

[0020] In general, in a first aspect, the invention features an
interferometry method for determining information about a test object,
including: directing test light to the test object positioned at a plane,
wherein one or more properties of the test light vary over a range of
incidence angles at the plane, the properties of the test light being
selected from the group consisting of the spectral content, intensity,
and polarization state; subsequently combining the test light with
reference light to form an interference pattern on a multi-element
detector so that different regions of the detector correspond to
different angles of the test light emerging from the test object, wherein
the test and reference light are derived from a common source; monitoring
the interference pattern using the multi-element detector while varying
an optical path difference between the test light and the reference
light; determining the information about the test object based on the
monitored interference pattern.

[0021] Implementations of the method can include one or more of the
following features. For example, the test object can include one or more
features and the variation of the one or more properties of the test
light are selected based on the one or more features of the test object.
The variation of the one or more properties of the test light can be
selected so that the information can be determined with higher
sensitivity relative to using test light for which the one or more
properties do not vary across the range of incident angles.

[0022] The method can include outputting information about the test
object.

[0023] The information about the test object can include information about
a refractive index of a layer of the test object. The information about
the test object can include information about a thickness of a layer of
the test object.

[0024] The test object can include one or more features and the
information about the test object can include information about the one
or more features. The information about the one or more features can
include a dimension (e.g., depth, height, width) of the one or more
features. The information about the one or more features can include
information about a relative position between two or more of the features
(e.g., overlay).

[0025] In some implementations, the method includes performing a
sensitivity analysis of the information and the one or more properties of
the test light can be selected based on the sensitivity analysis.

[0026] Directing the test light can include modulating the light so that
the intensity of the light varies over the range of incident angles.
Modulating the test light can include directing the test light through an
aperture corresponding to variation of the incident angles. Modulating
the test light can include diffracting the test light. The test light can
be modulated using a spatial light modulator (e.g., using an LCD or
micromirror array). Modulating the test light can include scanning light
into a range of light paths corresponding to different angles within the
range of incidence angles.

[0027] In general, in another aspect, the invention features an
interferometry method for determining information about a test object,
that includes: directing test light to the test object using a microscope
having an entrance pupil, wherein one or more properties of the test
light vary over the entrance pupil or a surface conjugate to the entrance
pupil, the properties of the test light being selected from the group
consisting of the spectral content, intensity, and polarization state;
subsequently combining the test light with reference light to form an
interference pattern on detector positioned at a surface conjugate to the
entrance pupil of the microscope, wherein the test and reference light
are derived from a common source; monitoring the interference pattern
using the detector while varying an optical path difference between the
test light and the reference light; and determining the information about
the test object based on the monitored interference pattern.

[0028] Implementations of the method can include one or more of the
following features and/or features of other aspects. For example, the
test light can form a pattern composed of discrete annular rings over the
entrance pupil or surface conjugate to the entrance pupil. Different
rings can have differing intensities. Alternatively, or additionally,
different rings can have different spectral composition.

[0029] The test light can form a pattern composed of discrete spots over
the entrance pupil or surface conjugate to the entrance pupil. Different
spots can have differing intensities and/or different spectral
composition.

[0030] In general, in a further aspect, the invention features an
interferometry method for determining information about a test object,
including: directing test light to the test object including one or more
features; subsequently combining the test light with reference light to
form an interference pattern on a multi-element detector so that
different regions of the detector correspond to different angles of the
test light emerging from the test object, wherein the test and reference
light are derived from a common source; monitoring the interference
pattern using the multi-element detector while varying an optical path
difference between the test light and the reference light; and
determining the information about the test object based on the monitored
interference pattern, wherein directing the test light includes selecting
a spectral content of the test light based on the features.

[0031] Implementations of the method can include one or more of the
following features and/or features of other aspects. For example,
directing the test light can include combining light from two or more
source elements to provide the selected spectral content. Directing the
test light can include filtering light from the common source to provide
the selected spectral content. Filtering the light can include varying
the intensity of the light at certain wavelengths relative to other
wavelengths.

[0032] In general, in a further aspect, the invention features an
apparatus that includes: a light source module; a scanning interferometer
positioned to receive light from the light source module and configured
to cause test light emerging from a test object positioned at a plane
over a range of angles to interfere with reference light on a detector so
that different regions of the detector correspond to different angles of
the test light emerging from the test object, wherein the test and
reference light are derived from the light source module and the light
source module is configured so that one or more properties of the test
light varies over a range of incidence angles at the plane, the
properties of the test light being selected from the group consisting of
the spectral content, intensity, and polarization state; and an
electronic processing module in communication with the detector, wherein
the apparatus is configured so that during operation the apparatus
monitors the interference pattern at the detector while the scanning
interferometer varies an optical path length between the test and
reference light and the electronic processing module determines
information about the test object based on the monitored interference
pattern. Embodiments of the apparatus can include one or more features of
other aspects.

[0033] In general, in another aspect, the invention features an apparatus
that includes: a light source module; a microscope having an entrance
pupil, the microscope being positioned to receive light from the light
source module and configured to cause test light emerging from a test
object to interfere with reference light on a detector, wherein the test
and reference light are derived from the light source module and the
light source module is configured so that one or more properties of the
test light varies over the entrance pupil or a plane conjugate to the
entrance pupil, the properties of the test light being selected from the
group consisting of the spectral content, intensity, and polarization
state; and an electronic processing module in communication with the
detector, wherein the apparatus is configured so that during operation
the apparatus monitors the interference pattern at the detector while the
scanning interferometer varies an optical path length between the test
and reference light and the electronic processing module determines
information about the test object based on the monitored interference
pattern.

[0034] Embodiments of the apparatus can include one or more of the
following features and/or features of other aspects. For example, the
light source module can include one or more light source elements and one
or more optical elements configured to selectively combine light having
differing spectral components from the light source elements. The light
source module can include one or more light source elements and one or
more filters to spectrally filter light from the light source elements.
The light source module can include one or more optical elements
configured to modulate an intensity profile of the test light in the
entrance pupil. The one or more optical elements can include a spatial
light modulator (e.g., an LCD or micromirror array). The one or more
optical elements can include a scanning element arranged to scan test
light to different locations in the entrance pupil. The one or more
optical elements can include a diffractive optical element configured to
diffract test light to modulate the intensity profile in the entrance
pupil.

[0035] The apparatus can include a translation stage configured to adjust
the relative optical path length between the test and reference light
when they form the interference pattern.

[0036] The apparatus can include a base for supporting the test object,
and wherein the translation stage is configured to move at least a
portion of the interferometer relative to the base.

[0037] The microscope can include a Mirau objective or a Linnik objective.
In general, a variety of different test objects can be studied using the
disclosed techniques. For example, test objects featuring complex surface
structure can be studied. Examples of complex surface structure include:
simple thin films (in which case, for example, the parameter(s) of
interest may be the film thickness, the refractive index of the film, the
refractive index of the substrate, or some combination thereof);
multilayer thin films; sharp edges and surface features that diffract or
otherwise generate complex interference effects; unresolved surface
roughness; unresolved surface features, for example, a sub-wavelength
width groove on an otherwise smooth surface; dissimilar materials (for
example, the surface may include a combination of thin film and a solid
metal, in which case the library may include both surface structure types
and automatically identify the film or the solid metal by a match to the
corresponding frequency-domain spectra); surface structure that give rise
to optical activity such as fluorescence; spectroscopic properties of the
surface, such as color and wavelength-dependent reflectivity;
polarization-dependent properties of the surface; and deflections,
vibrations or motions of the surface or deformable surface features that
result in perturbations of the interference signal.

[0038] The methods and techniques described herein can be used for
in-process metrology measurements of semiconductor chips. For example,
scanning interferometry measurements can be used for non-contact surface
topography measurements semiconductor wafers during chemical mechanical
polishing (CMP) of a dielectric layer on the wafer. CMP is used to create
a smooth surface for the dielectric layer, suitable for precision optical
lithography. Based on the results of the interferometric topography
methods, the process conditions for CMP (e.g., pad pressure, polishing
slurry composition, etc.) can be adjusted to keep surface
non-uniformities within acceptable limits.

[0039] As used herein, "light" is not limited to electromagnetic radiation
in the visible spectral region, but rather refers generally to
electromagnetic radiation in any of the ultraviolet, visible, near
infrared, and infrared spectral regions.

[0040] Unless otherwise defined, all technical and scientific terms used
herein have the same meaning as commonly understood by one of ordinary
skill in the art to which this invention belongs. In case of conflict
with any document incorporated by reference, the present disclosure
controls.

[0041] Other features and advantages will be apparent from the following
detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a schematic diagram of an embodiment of an interferometry
system.

[0043]FIG. 2 is a flowchart showing steps in a method for designing an
interferometry system including structured illumination.

[0044] FIG. 3 is a cross-sectional view of a model for a silicon grating
with period of 278 nm (single period shown) with three free parameters:
top CD (critical dimension), bottom CD, and depth. Axis labels are in
units of nm. Sensitivity analyses were performed for neighborhood of
depicted parameter values.

[0045] FIGS. 4(a)-(c) show Xenon source spectra with (a) no filtering; (b)
short-wave filtering with a cut-off wavelength of 600 nm; and (c)
short-wave filtering with a cut-off wavelength of 500 nm. Vertical lines
indicate optimum wavelength selections for subsequent analysis, given the
constraint of only three wavelength channels in total.

[0046] FIGS. 5(a)-(c) are plots showing sensitivity analysis for the
particular combination of the structure in FIG. 3, the source spectrum in
FIG. 4(a), and a constraint of using only 3 channels each for wavelength
and incident angle. Information content is depicted as a function of
wavelength and incident angle in FIG. 5(a), with lighter regions
indicating higher content; these data are collapsed as functions of
wavelength and angle in FIG. 5(b) and FIG. 5(c), respectively.

[0047]FIG. 6 shows a plot illustrating relative measurement repeatability
for the structure in FIG. 3 measured using the source spectra and trio of
wavelengths depicted in FIG. 4, showing monotonic improvement for all
parameters as unused wavelengths are filtered out.

[0048] FIG. 7(a) shows a perspective view of a single unit of a
multi-layer structure including a two-dimensional array of patterned
holes atop a buried grating. FIG. 7(b) shows a plot of sensitivity
results for the case where only top-layer parameters are measured; and
FIG. 7(c) shows a plot of sensitivity results for the case where only
buried-layer parameters are measured. Lighter regions in (b) and (c)
represent higher information content.

[0049] FIGS. 8(a) and 8(b) show plots of detector count spectral densities
having the same total counts but differing contributions from used
spectral channels. For FIG. 8(a), the source spectrum spans M=8 spectral
channels of width Bch, but only one of these is used. For FIG. 8(b),
the same total counts are allocated to the used channel, with no counts
in unused channels.

[0050] FIGS. 9(a) and 9(b) show sequences of plots comparing a
full-spectrum scan and a piece-wise narrowband scan, respectively, with
matched total measurement time Ttotal and total detector count rate
Ctotal per interval Ttotal/M.

[0051] FIG. 10(a) is a schematic diagram of an embodiment of an
interferometry system configured for single-frame pupil data acquisition.

[0052] FIG. 10(b) is a plot showing an intensity distribution at the
detector of the interferometry system shown in FIG. 10(a).

[0053] FIGS. 11(a) and 11(b) show plots of point-spread-function
broadening of a line spectrum for two cases of OPD range. For FIG. 11(a),
the OPD range is sufficiently long that PSF-broadened peaks remain
isolated with room to spare. For FIG. 11(b), the OPD is three times
smaller, broadening spectral-channel spacing and the PSF: however, the
PSF-broadened peaks remain distinct and undesired overlap is
substantively avoided.

[0054] FIG. 12 is a schematic diagram of an embodiment of an
interferometry system.

[0055]FIG. 13 is a diagram showing an illumination profile at a pupil
composed of discrete concentric rings of light, each corresponding to a
specific spectral line from a source (or sources). Gray levels indicate
different wavelengths, and white indicates non-illuminated regions.

[0056] FIGS. 14(a) and 14(b) are schematic diagrams showing embodiments of
assemblies that create laterally shifted images of a source point (with
more than one spectral component) in an entrance pupil of a microscope
objective.

[0057]FIG. 15 is a diagram showing an illumination profile at an exit
pupil of a microscope objective featuring discrete illumination angles
and a number of discrete wavelengths, indicated by gray level.

[0058] FIGS. 16(a) and 16(b) show an embodiment of an illumination
assembly at different times. The illumination assembly generates multiple
pupil illumination points of differing wavelengths in a time-multiplexed
manner. Illumination wavelength(s) can change between times t1 and
t2.

[0059]FIG. 17 is a schematic diagram showing components of a system that
combines spatially shaped pupil illumination with optimized polarization
elements.

[0060] FIG. 18 is a schematic diagram of an interferometry system showing
how various components of the system can be under automated control.

[0062]FIG. 20 is a schematic diagram of an embodiment of a LCD panel
composed of several layers.

[0063]FIG. 21 is a flowchart showing various steps in LCD panel
production.

[0064] Like reference numerals in different drawings refer to common
elements.

DETAILED DESCRIPTION

[0065] The complex reflectivity of a test object at multiple different
wavelengths can be measured using an interferometry system. For example,
FIG. 1 is a schematic diagram of an interferometry system 100, of the
type described in US Patent Publication No. 2006-0158659-A1
"INTERFEROMETER FOR DETERMINING CHARACTERISTICS OF AN OBJECT SURFACE" by
Xavier Colonna de Lega et. al., US Patent Publication No. 2006-0158658-A
"INTERFEROMETER WITH MULTIPLE MODES OF OPERATION FOR DETERMINING
CHARACTERISTICS OF AN OBJECT SURFACE", by Xavier Colonna de Lega et. al.,
and US Patent Publication No. 2006-0158657''A INTERFEROMETER FOR
DETERMINING CHARACTERISTICS OF AN OBJECT SURFACE, INCLUDING PROCESSING
AND CALIBRATION" by Xavier Colonna de Lega et. al., each of which is
incorporated herein by reference.

[0067] In the embodiment of FIG. 1, interference objective 106 is of the
Mirau-type, including an objective lens 118, beam splitter 120, and
reference surface 125. Beam splitter 120 separates input light 104 into
test light 122, which is directed to a test surface 124 of a test object
126, and reference light 128, which reflects from reference surface 125.
Objective lens 118 focuses the test and reference light to the test and
reference surfaces, respectively. The reference optic 130 supporting
reference surface 125 is coated to be reflective only for the focused
reference light, so that the majority of the input light passes through
the reference optic before being split by beam splitter 120.

[0068] After reflecting from the test and reference surfaces, the test and
reference light are recombined by beam splitter 120 to form combined
light 132, which is transmitted by beam splitter 112 and relay lens 136
to form an optical interference pattern on an electronic detector 134
(for example, a multi-element CCD or CMOS detector). The intensity
profile of the optical interference pattern across the detector is
measured by different elements of the detector and stored in an
electronic processor (not shown) for analysis. Unlike a conventional
profiling interferometer in which the test surface is imaged onto the
detector, in the present embodiment, relay lens 136 (e.g., a Bertrand
lens) images different points on the pupil plane 114 to corresponding
points on detector 134 (again as illustrating by dotted marginal rays 116
and solid chief rays 117).

[0069] Because each source point illuminating pupil plane 114 creates a
plane wave front for test light 122 illuminating test surface 124, the
radial location of the source point in pupil plane 114 defines the angle
of incidence of this illumination bundle with respect to the object
normal. Thus, all source points located at a given distance from the
optical axis correspond to a fixed angle of incidence, by which objective
lens 118 focuses test light 122 to test surface 124. A field stop 138
positioned between relay optic 108 and 110 defines the area of test
surface 124 illuminated by test light 122. After reflection from the test
and reference surfaces, combined light 132 forms a secondary image of the
source at pupil plane 114 of the objective lens. Because the combined
light on the pupil plane is then re-imaged by relay lens 136 onto
detector 134, the different elements of the detector 134 correspond to
the different illumination angles of test light 122 on test surface 124.

[0070] In some embodiments, polarization elements 140, 142, 144, and 146
are optionally included to define the polarization state of the test and
reference light being directed to the respective test and reference
surfaces, and that of the combined light being directed to the detector.
Depending on the embodiment, each polarization element can be a polarizer
(e.g., a linear polarizer), a retardation plate (e.g., a half or quarter
wave plate), or a similar optic that affects the polarization state of an
incident beam. Furthermore, in some embodiments, one or more of the
polarization elements can be absent. In some embodiment these elements
are adjustable, for instance mounted on a rotation mount, and even
motorized under electronic control of the system. Moreover, depending on
the embodiment, beam splitter 112 can be polarizing beam splitter or a
non-polarizing beam splitter. In general, because of the presence of
polarization elements 140, 142 and/or 146, the state of polarization of
test light 122 at test surface 124 can be a function of the azimuthal
position of the light in pupil plane 114.

[0071] In general, source 102 can be configured in a variety of ways as
described below. In conventional implementations, source 102 provides
illumination over a broad band of wavelengths (e.g., an emission spectrum
having a full-width, half-maximum of more than 50 nm, or preferably, even
more than 100 nm). For example, source 102 can be a white light emitting
diode (LED), a filament of a halogen bulb, an arc lamp such as a Xenon
arc lamp or a so-called supercontinuum source that uses non-linear
effects in optical materials to generate very broad source spectra (e.g.,
>200 nm). The broad band of wavelengths corresponds to a limited
coherence length.

[0072] A translation stage 150 adjusts the relative optic path length
between the test and reference light to produce an optical interference
signal at each of the detector elements. For example, in the embodiment
of the FIG. 1, translation stage 150 is a piezoelectric transducer
coupled to interference objective 106 to adjust the distance between the
test surface and the interference objective, and thereby vary the
relative optical path length between the test and reference light at the
detector. The scanning interferometry signals are recorded at detector
134 and processed by a computer 151 that is in communication with the
detector.

[0073] The scanning interferometry signal measured at each detector
element is analyzed by the computer, which is electronically coupled to
both detector 134 and translation stage 150. During analysis, computer
151 (or other electronic processor) determines the wavelength-dependent,
complex reflectivity of the test surface from the scanning interferometry
signal. For example, the scanning interferometry signal at each detector
element can be Fourier transformed to give the magnitude and phase of the
signal with respect to wavelength. This magnitude and phase can then be
related to conventional ellipsometry parameters.

Structured Illumination

[0074] In a PUPS measurement made using interferometry system 100 with a
conventional light source, a single detector element simultaneously
records signals corresponding to multiple source spectral components. The
signal level for a given spectral component thus only occupies a fraction
of the total dynamic range of the detector. However, the detection noise
is a result of fixed electronic noises (e.g., such as dark current) and
shot noise, which is proportional to the square root of the sum of all
signals occupying the dynamic range. Accordingly, the source can be
modified to utilize a subset of spatial and/or spectral components in
order to provide higher sensitivity to the parameters of interest of the
sample surface (e.g., say a film thickness or lateral dimension) than the
sensitivity provided by a broad spectral and spatial profile. In such
cases, there is benefit in detecting only the spectral and/or spatial
components that provide the higher level of sensitivity in the first
place. Accordingly, eliminating the other components from the source
spectrum allows increasing signal level for the useful channels, which
amounts to improving their signal to noise ratio (the detection noise
itself remains nominally the same if total signal remains the same).

[0075] The sections that follow describe various approaches of shaping
incident light to have particular configurations of spectral, angular,
and polarization content. Illumination profiles adapted in this way are
referred to generally herein as "structured illumination." The motivation
for doing so is predicated on having determined that such a configuration
is advantageous. Before describing specific approaches, it is instructive
to consider a general description of making such a determination, given a
set of possible tool configurations; a complex object; and the parameters
of the complex object to be measured.

[0076] Without wishing to be bound by theory, the overall signal detected
by a PUPS-capable tool measuring a complex object can be modeled as a
combination of constitutive signals Sj, each corresponding to light
with a particular combination of wavelength, incident angle, azimuthal
angle, and polarization, illuminating said complex object and returning
through an optional imaging analyzer having a particular polarization. In
turn, constitutive signals Sj can be determined using
electromagnetic simulation techniques such as rigorous coupled-wave
analysis (RCWA).

[0077] A process of designing a tool configuration to take advantage of
structured illumination is depicted in the flowchart shown in FIG. 2. The
first step is to determine the sensitivity of each constitutive signal
Sj to changes in each measured parameter pi of the complex
object. Sensitivity is generally considered to be a measure of how
responsive the detection apparatus/algorithm is to changes in a parameter
(e.g., a parameter related to the structure of the object under study). A
system having high sensitivity to a parameter would exhibit a
significant, measureable change in the system's response to a certain
change in parameter value. Conversely, low sensitivity means the system
would exhibit a small (e.g., undetectable) response to the same change in
parameter value. See, for example, W. Osten et al, "Simulations of
Scatterometry Down to 22 nm Structure Sizes and Beyond with Special
Emphasis on LER", AIP Conf. Proc., Sep. 28, 2009, Vol. 1173, pp. 371-378.

[0078] Sensitivity can be approximated, mathematically, as follows:

∂ S j ∂ p i ≈ S (
p i + dp i ) - S ( p i ) dp i , [ 1 ]
##EQU00001##

where pi is assigned a nominal expected value and S(p) can be
computed using an electromagnetic simulation technique such as RCWA.

[0079] Next, signal subsets should be defined for each tool configuration
under consideration. For example, it might be desired to consider all
combinations of M wavelengths and N incident angles, where M and N might
be determined by computational limitations. In any case, each signal
subset will have a corresponding signal sensitivity subset.

[0080] Each signal sensitivity subset can be combined with signal noise
levels to yield corresponding sets of parameter uncertainties, using
methods such as those described in the general linear least squares
section of W. Press et al, "Numerical Recipes in C, Second Edition
(1992)", Cambridge University Press, Chapter 15.4 General Linear Least
Squares, or, with particular regard to the current context, by Silver et
al in "Fundamental Limits of Optical Critical Dimension Metrology: A
Simulation Study", Proc. of SPIE Vol. 6518. The subset with the lowest
parameter uncertainties suggests the preferred signal subset, and hence
how the source should be shaped, for that particular complex object and
the particular measured parameters in question.

[0082] In general, the source spectrum can be shaped in a variety of ways.
In some embodiments, the source spectrum is shaped to contain a discrete
set of spectral components. Such a spectrum can be generated in a variety
of ways. For example, the light of multiple narrowband sources (e.g.,
LEDs, lasers, SLEDs) is combined using dichroic beamsplitters,
diffraction gratings, beam combiners, etc. In some embodiments, the light
from a broadband light source such as an arc lamp, LED, filament,
supercontinuum light source, is filtered using a monochromator, an
acousto-optic tunable filter, an LCD tunable filter, a Fabry-Perot
etalon, etc. Some of these components can switch rapidly enough that the
system can jump through multiple wavelengths during the integration of
each camera frame.

[0083] In certain embodiments, the system includes an optical element
(e.g., a grating or prism) that separates the spectral components
spatially and a micromirror array that controls the reflection of these
various spectral components toward or away from a recombining optical
element (another or the same grating or prism).

[0084] In some embodiments, spectral shaping can be performed using the
direct output of a mode-locked pulse laser.

[0085] By way of example, consider an optically unresolved grating etched
into silicon. FIG. 3 shows a cross-sectional view of such a structure. A
single period is shown along with the period and nominal values for top
width, bottom width, and etch depth. For this example, the period is
considered to be known and the metrology task is to measure the remaining
parameters.

[0086] Suppose that the total available spectrum is that of the unfiltered
Xenon source in FIG. 4(a), and that the computation/throughput budget
limits data usage to the combination of only three wavelengths and three
incident angles. This information, along with spectral/angular resolution
and a description of the structure in question, can be input to a
sensitivity simulator that subsequently outputs optimal selections of
wavelength and incident angles. Results for this example are shown in
FIGS. 5(a)-(c). The lighter regions of FIG. 5(a) indicate the most
information-rich combinations of wavelength and incident angle: given a
constraint of analyzing results for only three wavelength channels, one
could choose 480 nm, 500 nm, and 520 nm. These are indicated by the
vertical lines in FIG. 4(a).

[0087] For this example, wavelengths above this trio (and comprising the
bulk of the unfiltered Xe spectrum) contribute only noise and furthermore
occupy dynamic range within the detector that would be better occupied by
the wavelengths that are being used. This suggests filtering out the
unused longer wavelengths, as depicted in the progression of FIG. 4(b),
for which the recommended wavelength channels remain the same; and FIG.
4(c), for which the trio shifts to 460 nm, 480 nm, and 500 nm.

[0088] Simulations confirm the benefit of excluding unused wavelengths. As
shown in FIG. 6, predicted repeatability improves for all measured
parameters (top width, bottom width, and etch depth) for the progression
of spectra in FIGS. 4(a)-(c).

[0089] In the preceding example, the shorter wavelengths are the most
information-rich. However, this is not a general result, but rather
depends on the interplay between the source spectrum, structure geometry,
and the parameters of interest. Another example structure is shown in
FIG. 7(a), this time in the form of a two-dimensional periodic array of
barely-resolved patterned holes overlaying an optically unresolved buried
grating; only one periodic element is shown. Sensitivity analysis results
are shown in FIGS. 7(b) and 7(c) for different cases of measurement
parameters; note that the source spectrum in this case (not shown) spans
from about 420 nm to 620 nm.

[0090] For the case where one wishes to measure only top-layer parameters,
the simulation results of FIG. 7(a) indicate two isolated regions of
high-sensitivity wavelength/angle pairs. This suggests including
wavelengths of 450 nm and 560 nm in the source spectrum, and, if
computation/throughput constraints preclude using more than a pair of
wavelengths, excluding other wavelengths, including the band between the
used wavelengths.

[0091] FIG. 7(c) illustrates the impact of the choice of measurement
parameters: if only buried-film parameters are desired for this
particular structure, there is a single region of high sensitivity
centered at a wavelength of 450 nm.

[0092] Preferred spectral regions can also be influenced by the absorptive
properties of the layers of the structure being measured. Consider for
example a structure including features buried under a layer of
polysilicon, whose absorption coefficient k is relatively high
(>˜0.5) for wavelengths below ˜450 nm but substantially
lower (<˜0.1) above ˜550 nm. For a typical polysilicon
thickness in the ˜100-1000 nm range, the spectrum of light reaching
and returning from buried features will be heavily skewed towards
wavelengths above 500 nm, and these will likely be favored by sensitivity
analysis.

[0093] As demonstrated in FIG. 6, unused wavelengths in the source
spectrum may do worse than contributing nothing to performance: they can
actually worsen it. The cause for this can stem from shot noise
Nshot, a statistical phenomenon whose contribution to a given
spectral channel scales as product of the channel bandwidth Bch and
the square root of the total counts seen by the detector Ctotal:

Nshot=KBch {square root over (Ctotal)}, [2]

where K is a proportionality constant.

[0094] FIG. 8 shows plots of two detector count spectral distributions
having the same total count intensity, as might be the case where source
intensity is adjusted to take full advantage of detector range. In the
first case, FIG. 8(a), only a fraction 1/M of the total power lies within
the used channel bandwidth, whereas in the second case, FIG. 8(b), all
power lies therein. For both cases, the shot noise will be the same, as
given by equation [2]. However, the signal-to-noise ratio (SNR) is M
times larger for case (b) than case (a):

[0095] For simplicity, this example includes power spectral distributions
that are constant across their extent. However, in general, similar
principles apply for other spectral distributions.

[0096] In some embodiments, the PUPS measurement includes a series of
individual OPD (optical path difference) scans performed each with a
different narrowband source spectrum. In this case the scan length can be
shorter than the coherence length and a number of PSI (phase-shifting
interferometry) algorithms can be applied for the spectral analysis. This
potentially provides improved signal-to-noise ratio for each wavelength
for the same overall data acquisition duration. The multiple wavelengths
can be generated using one of the methods listed above: multiple sources
alternately turned on/off or shuttered, switchable sources (for instance
multiple LED devices on a carousel with one LED at a time in a position
to illuminate the object surface), etc. Data analysis (measurement of
interference signal amplitude and phase) can be performed, for example,
using a phase-shifting algorithm.

[0097] The potential benefit of performing sequential narrowband OPD scans
can be explained by comparing the scanning schemes depicted in FIGS. 9(a)
and 9(b). For the first scheme, depicted in the plot sequence in FIG.
9(a), a broadband spectrum spanning M spectral channels is used over the
total measurement time Ttotal. The second scheme, depicted in the
plot sequence in FIG. 9(b), includes M consecutive narrowband scans, each
of duration Ttotal/M and each addressing a different spectral
channel.

[0098] Parameters for each scheme are compared in Table 1, below. Both
share the same total measurement time Ttotal and total detector
count rate (Ctotal per interval Ttotal/M), with the latter
typically chosen to exploit the full detector range. For the
full-spectrum scheme, channels accumulate counts at a slower rate but
over a longer span, whereas for the narrowband scheme, channels
accumulate counts at a faster rate but over a shorter span.

[0099] These differences balance to yield the same total counts per
channel: on the face of it, this might seem to suggest no advantage of
one scheme over the other. However, shot noise scales with the square
root of total counts over all channels, as in equation [2], yielding a
{square root over (M)} advantage for the piece-wise narrowband scheme
spectrum in terms of both shot noise and SNR.

[0100] For the sake of simplicity in the preceding illustration, count
rate is depicted as uniform across all channels and implicitly uniform
over the scan duration, but similar principles apply even with
non-uniform count spectra and time-varying count rates.

[0101] In some embodiments, the individual wavelength measurements are
each performed with a single frame of data, i.e., with no scanning
required. FIG. 10(a) shows an optical configuration 1000 where the source
is spatially coherent and monochromatic. Specifically, system 1000
includes a fiber-coupled laser source 1010 (although other coherent
sources may be used). Illumination optics, including lenses 1012, 1014
and 1016, create a plane wavefront in the entrance pupil of a
non-interferometric objective. Part of the illumination light also goes
to a reference mirror 1020. The light reflected from the sample and from
mirror 1020 interferes on the detector array 134. Mirror 1020 is tilted
in order to create a dense spatial carrier pattern onto the detector, as
seen in FIG. 10(b). These data are processed using spatial carrier
techniques, either in the space or frequency domains. Multiple frames can
be captured for each wavelength in order to reduce noise. The system
shown in FIG. 10(a) may beneficially have low sensitivity to vibration
(e.g., since data can be acquired from a single frame). Polarizing
elements can also be included on both test and reference legs in order to
independently control the polarization of the illumination light.

[0102] As mentioned previously, in some embodiments, spectral shaping can
be achieved by combining a broadband light source with one or more
filters, such as tunable filters. A benefit of combining broadband light
sources with tunable filters is the ability to pick optimum wavelengths
for different metrology applications. In some embodiments, a computer
performs a sensitivity analysis of a model of the nominal object
structure, determines the optimum wavelengths to be used and sets the
tunable filters (or other means of spectral selection) accordingly before
data acquisition. The relative strengths of the wavelengths used may also
be adjusted in accordance with the results of sensitivity analysis, e.g.,
higher power contribution for wavelengths affording higher sensitivity.

[0103] In certain embodiments, all available wavelengths are used with
relative power contribution adjusted in accordance with the results of
sensitivity analysis. For example, tunable filters could be used with a
broadband source to produce a spectrum with higher power at wavelengths
associated with higher sensitivity. This approach can offer advantages in
cases where it is tenable to exploit most or all available wavelengths in
the analysis, or in cases where information content is widely distributed
as a function of wavelength: for example, if one is seeking to fit many
model parameters with competing demands on preferred spectral range. This
is also beneficial when sensitivity analysis shows that the (wavelength,
incident angle) positions of maximum sensitivity move substantially with
variations of the structure parameters (within the process window).

[0104] Another benefit of using discrete wavelength bands is that it
becomes possible to avoid mixing spectral components as a result of the
spectral analysis. For example, for a source spectrum comprising discrete
spectral lines, the Fourier transform of the interference signal will be
the convolution of the individual lines with a point-spread-function
(PSF) whose width depends on the range of optical path difference (OPD)
variation in the interferometer during data acquisition, as depicted in
FIGS. 11(a) and 11(b).

[0105] It is straightforward to compute the OPD range required to avoid
overlap of the convolved PSFs in the spectral domain: FIG. 11(b) depicts
the case where this is applied to its limit, i.e., where OPD is just long
enough to keep spectral contributions distinct. The spectral analysis is
then conducted only for the known spectral components. Eliminating mixing
increases the accuracy of the measurement process and potentially
simplifies modeling, whereas the limitation to a finite set of useful
wavelengths increases the signal to noise of the measurement process and
consequently its repeatability.

[0106] Spatial Shaping

[0107] In some embodiments, the distribution of light at the
interferometer pupil is spatially shaped so that the object surface is
illuminated only at specific angles of incidence and/or azimuthal
positions. For instance the illumination pattern at the pupil is a set of
concentric rings, or a set of radial lines, or a set of discrete points
or other combinations.

[0108] Spatial shaping can be performed in a variety of ways. For example,
patterns can be generated using:

[0112] (iv) programmable LCDs or micro-mirror modulators that act as a
dynamic diffraction grating creating the required light distribution in
the pupil plane;

[0113] (v) a flying spot that is scanned over the pupil at high speed: the
output of a mono-mode fiber is reimaged onto the pupil via one or two
scanning mirrors; shuttering or turning the source on or off or generally
attenuating the spot during the scan creates the desired (reprogrammable)
pattern at the pupil. The entire illumination pattern is scanned over the
pupil at least once per camera frame. In certain embodiments, an XY
scanner is built using acousto-optic modulators (see below); and/or

[0114] (vi) a non-imaging device, such as a glass rod that is illuminated
with a source point (e.g., exit face of a mono-mode fiber), creates at
its output a conical illumination pattern.

[0115] A benefit of this approach is an improvement in the accuracy of the
instrument since angles of incidence or azimuthal positions can be known
as a result of the spatial source shaping.

[0116] A further benefit is found when the imaging optics that relay the
exit pupil of the interferometer onto the detector are defocused by a
controllable amount. In this case a single illumination point created in
the pupil is re-imaged as a blurred spot onto the camera. All the pixels
covered by the blur spot receive light that corresponds to a specific
angle of incidence and azimuthal position. It follows that the
information they collect can be combined (or binned) without loss of
accuracy, creating a sort of super-pixel. The benefit is an increased
signal-to-noise ratio of the resulting measurement point since more
photons can now be effectively captured by the detector, assuming source
intensity can be increased.

[0117] Discrete pupil points can also enable the use of a photodiode array
instead of a high resolution camera. Photo diode arrays are known to have
better noise statistics and can potentially be run at much higher speeds.
The preferred configuration of this embodiment has one photodiode element
for each pupil illumination point.

[0118] Referring to FIG. 12, in some embodiments, it is possible to reduce
diffraction-induced mixing of light coming from different illumination
directions. For example, referring to FIG. 12, a system 1200 includes an
effective field stop 1210 in the system's imaging leg rather than, or in
addition to, field stop 138 in the illumination leg. Thereby, a
diffraction-limited size of every discrete pupil illumination point can
be produced Since pupil locations are transformed into illumination
angles, the angular range of light hitting the sample (from any pupil
illumination point) is correspondingly reduced.

[0119] Field stop 1210 is position at a conjugate plane to the sample
plane. Field stop 120 blocks light coming from areas outside the test
pad. Camera 134 is placed in a conjugate pupil plane or in a plane nearby
(134'), leading to a slight blurring of the pupil image. Diffraction at
effective imaging field stop 1210 further blurs the pupil illumination
points on the camera, which does not impose a problem in this measurement
mode with discrete pupil illumination points as long as the pupil point
images do not overlap on the camera.

[0120] Spectral and Spatial Shaping

[0121] In some embodiments, two or more discrete spectral lines of a light
source are displaced spatially in an interferometer pupil so individual
detector elements detect light corresponding to single spectral
components or unique combinations of different spectral components. For
example, in certain embodiments, illumination at the pupil can be
composed of multiple monochromatic concentric rings, as shown in FIG. 13.
Such configurations can provide optimum signal to noise ratio for the
measurement performed at each wavelength while allowing collecting data
simultaneously at multiple wavelengths.

[0122] In general, such pupil illumination profiles can be generated in a
variety of ways. For example, a refracting element, such as a specially
designed lens (e.g., a compound or single-element lens), that introduces
significant amounts of lateral color when imaging a source point (ring or
point or line segment) onto the pupil plane can be used. Alternatively,
or additionally, one can use a diffractive element that performs the same
function.

[0123] Referring to FIG. 14(a), an assembly 1400 for generating a spectral
distribution at a pupil plane includes lenses 1410 and 1420 positioned to
direct light from a source plane 1401 to an entrance pupil 1402 of a
microscope objective. Light rays are shown for a single source point.
Assembly 1400 also includes a field stop 1430 positioned in the light
path between lenses 1410 and 1420. The source point emits light at
multiple discrete wavelengths. The assembly disperses the light from this
point to different positions at pupil plane 1402. The layout of the
assembly preserves telecentricity of illumination (i.e., the chief ray is
parallel to the optical axis) in order to preserve the size of the field
stop in the object space of the microscope objective.

[0124] In some embodiments, as illustrated in FIG. 14(a), assembly 1400
provides significant lateral color with negligible longitudinal color
(i.e., dispersion to different points in pupil plane 1402 but to
substantially the same location along the optical axis).

[0125] In certain embodiments, assembly 1400 can include one or more
additional components, such as a diffracting element 1440. Examples of
diffracting elements that can be used in such assemblies include gratings
that have concentric grooves of equal pitch. The groove profile can be
designed to provide maximum diffraction efficiency in the diffraction
order that passes through the field stop. Other diffraction orders,
including the 0th order are blocked by field stop 1430.

[0126] In some embodiments, wavelength spectra can be shaped as a function
of incident angle and/or azimuthal angle in accordance with the results
of a sensitivity analysis for these parameters. This approach can offer
advantages in cases where it is tenable to exploit most or all available
experimental data in the analysis, e.g., using a functional fit through
simulated data. Such embodiments can offer benefits in cases where
information content is widely distributed as a function of wavelength,
incident angle, and azimuthal angle: for example, if one is seeking to
fit many model parameters with competing demands on preferred ranges of
these parameters.

[0127]FIG. 15 shows an example of a pupil illumination that uses spatial
and spectral shaping. Here, the exit pupil of the objective is imaged
with a slight defocus leading to enlarged spots on the camera (as shown).
Each illumination spot is composed of light having a specific spectral
profile, in this example one of three available wavelengths.

[0128] Illumination patterns of the kind shown in FIG. 15 can be formed in
a variety of ways. For example, referring to FIGS. 16(a) and 16(b), such
illumination patterns can be achieved using an assembly 1600 that
includes multiple monochromatic light sources (not shown) which are
coupled into a fiber waveguide. Waveguide 1610 directs the light to a
collimating lens 1612, which collimates the light and directs it to a
beamsplitter 1615. Beamsplitter 1615 directs the light to a dynamic
diffraction grating 1620 (e.g., a micromirror modulator), which diffracts
at least a portion of the incident light back to beamsplitter 1615. The
beamsplitter transmits light to a lens 1614, which focuses light onto an
entrance pupil of the interference microscope (not shown).

[0129] Each of the monochromatic light sources can be intensity modulated
(e.g., using a shutter or by modulating the current used to power the
light sources). During an active time of each frame of the microscope's
camera, light should be directed to each illumination spot at least once
using the corresponding light source. This can be done by flashing the
light sources at different times (t1, t2, . . . ) and
concurrently providing a diffraction grating generated by the micro
mirror modulator that directs the beam to the desired pupil locations
(one or multiple at a time). FIGS. 16(a) and 16(b) show illumination at
times t1 and t2, respectively, illuminating different locations
in pupil 1602.

[0130] Static solutions with static DOEs and permanent illumination with
multiple wavelengths is also possible. In such implementations,
dispersion of the diffractive elements can be used to separate the
colors.

[0131] Spatial, Spectral and Polarization Shaping

[0132] In some embodiments, sub-regions of the pupil illumination are
polarized differently and/or sub-regions of the detector are analyzed
differently in an effort to maximize the information content of a
measurement. A sensitivity analysis may be used to determine the optimal
scheme of sub-region patterns with different polarization states.
Polarization elements can be placed in or near conjugate pupil planes in
the illumination and imaging leg of the interferometer or in or near the
pupil plane of the interference objective. Polarizer and analyzer
patterns can be static or dynamic. Dynamic patterns can be changed to
provide optimized sensitivity for multiple different applications.
Dynamic changes of the polarizing elements can be achieved using
mechanically interchangeable elements (e.g., sliders or filter wheels
equipped with an assortment of patterns) or electrically addressable
elements (e.g., liquid crystal based spatial light modulators).

[0133] In some embodiments, optimized polarizer/analyzer patterns are
combined with spatial and/or spectral shaping of the pupil illumination.
For example, referring to FIG. 17, a system 1700 can combine spatial
shaping of the pupil illumination with optimized polarizer/analyzer
patterns. Here, system 1700 includes a microlens array 1710 that
generates an array of illumination points, each of which has its
dedicated cell in an illumination polarization array 1720 and its
dedicated cell in an imaging analyzer array 1730.

Alternative Embodiments

[0134] While the foregoing description considers a variety of
interferometry systems, other implementations are also possible.
Generally, the techniques disclosed herein can be applied to variations
of interferometry system 100. For example, while the interference
microscope shown in FIG. 1 is a Mirau-type microscope, other types of
microscope can also be used. For example, in some embodiments, a
Linnik-type interference microscope can be used. In certain embodiments,
a Linnik-type microscope can provide more flexibility for modulating
polarization of the reference beam because the reference beam path is
physically more accessible relative to a Mirau-type objective. A
quarter-wave plate in the collimated space of the reference path, for
example, can be provided to cause a rotation of the polarization in
double-pass and therefore provide a completely illuminated pupil as seen
by the camera. The use of a Linnik-type interference microscope can also
allow adjusting the reference light intensity with respect to the test
light intensity in order to maximize the fringe contrast. For example, a
neutral density filter can be positioned in the path of the reference
light to reduce its intensity as necessary.

[0135] Adjustment of the reference light intensity relative to the test
light intensity can also be done with a polarized Mirau objective, e.g.,
in which the beam splitter is sandwiched between two quarter wave plates.
In such configurations, the reference and test light have orthogonal
polarization states. Placing an analyzer aligned with the reference light
polarization (lighting the entire pupil) can cause the test light to
experience a dissimilar polarizer/analyzer configuration.

[0136] Furthermore, interferometry systems used for reflectivity
measurements can, in some embodiments, be used for other types of
metrology as well. For example, interferometry system 100 can be used for
surface profiling measurements in addition to reflectivity measurements.
In some embodiments, interferometry systems can also be adapted for
additional functionality by switching between various hardware
configurations. For example, the system hardware can be switched between
conventional SWLI imaging and PUPS imaging, allowing, e.g., surface
profile measurements to be made alongside reflectivity measurements.

[0137] FIG. 18 shows a schematic diagram of how various components in
interferometry system 100 can be automated under the control of
electronic processor 970, which, in the presently described embodiment,
can include an analytical processor 972 for carrying out mathematical
analyses, device controllers 974 for controlling various components in
the interferometry system, a user interface 976 (e.g., a keyboard and
display), and a storage medium 978 for storing calibration information,
data files, a sample models, and/or automated protocols.

[0138] First, the system can include a motorized turret 910 supporting
multiple objectives 912 and configured to introduce a selected objective
into the path of input light 104. One or more of the objectives can be
interference objectives, with the different interference objectives
providing different magnifications. Furthermore, in certain embodiments,
one (or more) of the interference objectives can be especially configured
for the ellipsometry mode (e.g., PUPS mode) of operation by having
polarization element 146 (e.g., a linear polarizer) attached to it. The
remaining interference objectives can be used in the profiling mode and,
in certain embodiments, can omit polarization element 146 so as to
increase light efficiency (such as for the embodiment described above in
which beam splitter 112 is a polarizing beam splitter and polarization
element is 142 is a quarter wave plate). Moreover, one or more of the
objectives can be a non-interferometric objective (i.e., one without a
reference leg), each with a different magnification, so that system 100
can also operate in a conventional microscope mode for collecting optical
images of the test surface (in which case the relay lens is set to image
of test surface to the detector). Turret 910 is under the control of
electronic processor 970, which selects the desired objective according
to user input or some automated protocol.

[0139] Next, the system includes a motorized stage 920 (e.g., a tube lens
holder) for supporting relay lenses 136 and 236 and selectively
positioning one of them in the path of combined light 132 for selecting
between the first mode (e.g., an ellipsometry or reflectometry mode) in
which the pupil plane 114 is imaged to the detector and the second mode
(e.g., profiling/overlay or microscope mode) in which the test surface is
imaged to the detector. Motorized stage 920 is under the control of
electronic processor 970, which selects the desired relay lens according
to user input or some automated protocol. In other embodiments, in which
a translation stage is moved to adjust the position of the detector to
switch between the first and second modes, the translation is under
control of the electronic processor. Furthermore, in those embodiments
with two detection channels, each detector is coupled to the electronic
processor 970 for analysis.

[0140] Furthermore, the system can include motorized apertures 930 and 932
under control of electronic processor 970 to control the dimensions of
field stop 138 and aperture stop 115, respectively. Again the motorized
apertures are under the control of electronic processor 970, which
selects the desired settings according to user input or some automated
protocol.

[0141] Also, translation stage 180, which is used to vary the relative
optical path length between the test and reference legs of the
interferometer, is under the control of electronic processor 970. As
described above, the translation stage can be coupled to adjust the
position of the interference objective relative to a mount 940 for
supporting test object 126. Alternatively, in further embodiments, the
translation stage can adjust the position of the interferometry system as
a whole relative to the mount, or the translation stage can be coupled to
the mount, so it is the mount that moves to vary the optical path length
difference.

[0142] Furthermore, a lateral translation stage 950, also under the
control of electronic processor 970, can be coupled to the mount 940
supporting the test object to translate laterally the region of the test
surface under optical inspection. In certain embodiments, translation
stage 950 can also orient mount 940 (e.g., provide tip and tilt) so as to
align the test surface normal to the optical axis of the interference
objective.

[0143] Finally, an objective handling system 960, also under control of
electronic processor 970, can be coupled to mount 940 to provide
automated introduction and removal of test samples into system 100 for
measurement. For example, automated wafer handling systems known in the
art can be used for this purpose. Furthermore, if necessary, system 100
and object handling system can be housed under vacuum or clean room
conditions to minimize contamination of the test objects.

[0144] The resulting system provides great flexibility for providing
various measurement modalities and procedures. For example, the system
can first be configured in the microscope mode with one or more selected
magnifications to obtain optical images of the test object for various
lateral positions of the object. Such images can be analyzed by a user or
by electronic processor 970 (using machine vision techniques) to identify
certain regions (e.g., specific structures or features, landmarks,
fiducial markers, defects, etc.) in the object. Based on such
identification, selected regions of the sample can then be studied in the
ellipsometry mode to determine sample properties (e.g., refractive index,
underlying film thickness(es), material identification, etc.).

[0145] Accordingly, the electronic processor causes stage 920 to switch
the relay lens to the one configured for the ellipsometry mode and
further causes turret 910 to introduce a suitable interference objective
into the path of the input light. To improve the accuracy of the
ellipsometry measurement, the electronic processor can reduce the size of
the field stop via motorized aperture 930 to isolate a small laterally
homogenous or periodic region of the object. After the ellipsometry
characterization is complete, electronic processor 970 can switch the
instrument to the profiling mode, selecting an interference objective
with a suitable magnification and adjusting the size of field stop
accordingly. The profiling/overlay mode captures interference signals
that allow reconstructing the topography of, for example, one or more
interfaces that constitute the object. Notably, the knowledge of the
optical characteristics of the various materials determined in the
ellipsometry mode allows for correcting the calculated topography for
thin film or dissimilar material effects that would otherwise distort the
profile. See, for example, U.S. patent application Ser. No. 10/795,579
entitled "PROFILING COMPLEX SURFACE STRUCTURES USING SCANNING
INTERFEROMETRY" and published as U.S. Patent Publication No.
US-2004-0189999-A1, which is incorporated by reference. If desired, the
electronic processor can also adjust the aperture stop diameter via
motorized aperture 932 to improve the measurement in any of the various
modes.

[0146] When used in conjunction with automated object handling system 960,
the measurement procedure can be repeated automatically for a series of
samples. This could be useful for various process control schemes, such
as for monitoring, testing, and/or optimizing one or more semiconductor
processing steps.

[0147] For example, the system can be used in a semiconductor process for
tool-specific monitoring or for controlling the process flow itself. In
the process-monitoring application, single/multi-layer films are grown,
deposited, polished, or etched away on unpatterned Si wafers (monitor
wafers) by the corresponding process tool and subsequently the thickness
and/or optical properties are measured using the interferometry system
disclosed herein (for example, by using the ellipsometry mode, the
profiling/overlay mode, or both). The average, as well as within-wafer
uniformity, of thickness (and/or optical properties) of these monitor
wafers are used to determine whether the associated process tool is
operating with targeted specification or should be retargeted, adjusted,
or taken out of production use.

[0148] In the process control application, latter single/multi-layer films
are grown, deposited, polished, or etched away on patterned production
wafers by the corresponding process tool and subsequently the thickness
and/or optical properties are measured with the interferometry system
disclosed herein (for example, by using the ellipsometry mode, the
profiling mode, or both). Production measurements used for process
control typical include a small measurement site and the ability to align
the measurement tool to the sample region of interest. This site may
consist of a multi-layer film stack (that may itself be patterned) and
thus requires complex mathematical modeling in order to extract the
relevant physical parameters. Process control measurements determine the
stability of the integrated process flow and determine whether the
integrated processing should continue, be retargeted, redirected to other
equipment, or shut down entirely.

[0149] Specifically, for example, the interferometry system disclosed
herein can be used to monitor the following equipment: diffusion, rapid
thermal anneal, chemical vapor deposition tools (both low pressure and
high pressure), dielectric etch, chemical mechanical polishers, plasma
deposition, plasma etch, lithography track, and lithography exposure
tools. Additionally, the interferometry system disclosed herein can be
used to control the following processes: trench and isolation, transistor
formation, as well as interlayer dielectric formation (such as dual
damascene).

[0150] In general, a variety of different light sources can be used to
provide structured illumination. For example, the light source may be any
of: an incandescent source, such as a halogen bulb or metal halide lamp,
with or without spectral bandpass filters; a broadband laser diode; a
light-emitting diode; a supercontinuum light source (as mentioned above);
a combination of several light sources of the same or different types; an
arc lamp; any source in the visible spectral region; any source in the IR
spectral region, particularly for viewing rough surfaces & applying phase
profiling; and any source in the UV spectral region, particularly for
enhanced lateral resolution. For broadband applications, the source
preferably has a net spectral bandwidth broader than 5% of the mean
wavelength, or more preferably greater than 10%, 20%, 30%, or even 50% of
the mean wavelength. For tunable, narrow-band applications, the tuning
range is preferably broad (e.g., greater than 50 nm, greater than 100 nm,
or greater than even 200 nm, for visible light) to provide reflectivity
information over a wide range of wavelengths, whereas the spectral width
at any particular setting is preferable narrow, to optimize resolution,
for example, as small as 10 nm, 2 nm, or 1 nm. The source may also
include one or more diffuser elements to increase the spatial extent of
the input light being emitted from the source.

[0151] Furthermore, the various translations stages in the system, such as
translation stage 150, may be: driven by any of a piezo-electric device,
a stepper motor, and a voice coil; implemented opto-mechanically or
opto-electronically rather than by pure translation (e.g., by using any
of liquid crystals, electro-optic effects, strained fibers, and rotating
waveplates) to introduce an optical path length variation; any of a
driver with a flexure mount and any driver with a mechanical stage, e.g.
roller bearings or air bearings.

[0152] The electronic detector can be any type of detector for measuring
an optical interference pattern with spatial resolution, such as a
multi-element CCD or CMOS detector.

[0153] The analysis steps described above can be implemented in computer
programs using standard programming techniques. Such programs are
designed to execute on programmable computers or specifically designed
integrated circuits, each comprising an electronic processor, a data
storage system (including memory and/or storage elements), at least one
input device, and least one output device, such as a display or printer.
The program code is applied to input data (e.g., scanning interference
signals from the detector) to perform the functions described herein and
generate output information (e.g., overlay error, refractive index
information, thickness measurement(s), surface profile(s), etc.), which
is applied to one or more output devices. Each such computer program can
be implemented in a high-level procedural or object-oriented programming
language, or an assembly or machine language. Furthermore, the language
can be a compiled, interpreted or intermediate language. Each such
computer program can be stored on a computer readable storage medium
(e.g., CD ROM or magnetic diskette) that when read by a computer can
cause the processor in the computer to perform the analysis and control
functions described herein.

[0154] Interferometry metrology systems, such as those discussed
previously, can be used in the production of integrated circuits to
monitor and improve overlay between patterned layers. For example, the
interferometry systems and methods can be used in combination with a
lithography system and other processing equipment used to produce
integrated circuits. In general, a lithography system, also referred to
as an exposure system, typically includes an illumination system and a
wafer positioning system. The illumination system includes a radiation
source for providing radiation such as ultraviolet, visible, x-ray,
electron, or ion radiation, and a reticle or mask for imparting the
pattern to the radiation, thereby generating the spatially patterned
radiation. In addition, for the case of reduction lithography, the
illumination system can include a lens assembly for imaging the spatially
patterned radiation onto the wafer. The imaged radiation exposes resist
coated onto the wafer. The illumination system also includes a mask stage
for supporting the mask and a positioning system for adjusting the
position of the mask stage relative to the radiation directed through the
mask. The wafer positioning system includes a wafer stage for supporting
the wafer and a positioning system for adjusting the position of the
wafer stage relative to the imaged radiation. Fabrication of integrated
circuits can include multiple exposing steps. For a general reference on
lithography, see, for example, J. R. Sheats and B. W. Smith, in
Microlithography: Science and Technology (Marcel Dekker, Inc., New York,
1998), the contents of which is incorporated herein by reference.

[0155] As is well known in the art, lithography is a critical part of
manufacturing methods for making semiconducting devices. For example,
U.S. Pat. No. 5,483,343 outlines steps for such manufacturing methods.
These steps are described below with reference to FIGS. 19(a) and 19(b).
FIG. 19(a) is a flow chart of the sequence of manufacturing a
semiconductor device such as a semiconductor chip (e.g., IC or LSI), a
liquid crystal panel or a CCD. Step 1151 is a design process for
designing the circuit of a semiconductor device. Step 1152 is a process
for manufacturing a mask on the basis of the circuit pattern design. Step
1153 is a process for manufacturing a wafer by using a material such as
silicon.

[0156] Step 1154 is a wafer process, which is called a pre-process
wherein, by using the so prepared mask and wafer, circuits are formed on
the wafer through lithography. To form circuits on the wafer, patterns
from multiple masks are sequentially transferred to different layers on
the wafer, building up the circuits. Effective circuit production
requires accurate overlay between the sequentially formed layers. The
interferometry methods and systems described herein can be especially
useful to provide accurate overlay and thereby improve the effectiveness
of the lithography used in the wafer process.

[0157] Step 1155 is an assembling step, which is called a post-process
wherein the wafer processed by step 1154 is formed into semiconductor
chips. This step includes assembling (dicing and bonding) and packaging
(chip sealing). Step 1156 is an inspection step wherein operability
check, durability check and so on of the semiconductor devices produced
by step 1155 are carried out. With these processes, semiconductor devices
are finished and they are shipped (step 1157).

[0158] FIG. 19(b) is a flow chart showing details of the wafer process.
Step 1161 is an oxidation process for oxidizing the surface of a wafer.
Step 1162 is a CVD process for forming an insulating film on the wafer
surface. Step 1163 is an electrode forming process for forming electrodes
on the wafer by vapor deposition. Step 1164 is an ion implanting process
for implanting ions to the wafer. Step 1165 is a resist process for
applying a resist (photosensitive material) to the wafer. Step 1166 is an
exposure process for printing, by exposure (i.e., lithography), the
circuit pattern of the mask on the wafer through the exposure apparatus
described above. Once again, as described above, the use of the
interferometry systems and methods described herein can improve the
accuracy and resolution of such lithography steps.

[0159] Step 1167 is a developing process for developing the exposed wafer.
Step 1168 is an etching process for removing portions other than the
developed resist image. Step 1169 is a resist separation process for
separating the resist material remaining on the wafer after being
subjected to the etching process. By repeating these processes, circuit
patterns are formed and superimposed on the wafer.

[0160] As mentioned previously, the interferometry systems and methods
disclosed herein can be used in the manufacture of flat panel displays
such as, for example, liquid crystal displays (LCDs).

[0161] In general, a variety of different LCD configurations are used in
many different applications, such as LCD televisions, desktop computer
monitors, notebook computers, cell phones, automobile GPS navigation
systems, automobile and aircraft entertainment systems to name a few.
While the specific structure of a LCD can vary, many types of LCD utilize
a similar panel structure. Referring to FIG. 20, for example, in some
embodiments, a LCD panel 450 is composed of several layers including two
glass plates 452, 453 connected by seals 454. Glass plates 452 and 453
are separated by a gap 464, which is filled with a liquid crystal
material. Polarizers 456 and 474 are applied to glass plates 453 and 452,
respectively. One of the polarizers operates to polarize light from the
display's light source (e.g., a backlight, not shown) and the other
polarizer serves as an analyzer, transmitting only that component of the
light polarized parallel to the polarizer's transmission axis.

[0162] An array of color filters 476 is formed on glass plate 453 and a
patterned electrode layer 458 is formed on color filters 476 from a
transparent conductor, commonly Indium Tin Oxide (ITO). A passivation
layer 460, sometimes called hard coat layer, based on SiOx is coated over
the electrode layer 458 to electrically insulate the surface. Polyimide
462 is disposed over the passivation layer 460 to align the liquid
crystal fluid 464.

[0163] Panel 450 also includes a second electrode layer 472 formed on
glass plate 452. Another hard coat layer 470 is formed on electrode layer
472 and another polyimide layer 468 is disposed on hard coat layer 470.
In active matrix LCDs ("AM LCDs"), one of the electrode layers generally
includes an array of thin film transistors (TFTs) (e.g., one or more for
each sub-pixel) or other integrated circuit structures.

[0164] The liquid crystal material is birefringent and modifies the
polarization direction of the light propagating through the material. The
liquid crystal material also has a dielectric anisotropy and is therefore
sensitive to electric fields applied across gap 464. Accordingly, the
liquid crystal molecules change orientation when an electric field is
applied, thereby varying the optical properties of the panel. By
harnessing the birefringence and dielectric anisotropy of the liquid
crystal material, one can control the amount of light transmitted by the
panel.

[0165] The cell gap Δg, i.e., thickness of the liquid crystal layer
464, is determined by spacers 466, which keep the two glass plates 452,
453 at a fixed distance. In general, spacers can be in the form of
preformed cylindrical or spherical particles having a diameter equal to
the desired cell gap or can be formed on the substrate using patterning
techniques (e.g., conventional photolithography techniques).

[0166] In general, LCD panel manufacturing involves multiple process steps
in forming the various layers. For example, referring to FIG. 21, a
process 499 includes forming the various layers on each glass plate in
parallel, and then bonding the plates to form a cell. The cell is then
filled with the liquid crystal material and sealed. After sealing, the
polarizers are applied to the outer surface of each of the glass plates,
providing the completed LCD panel.

[0167] In general, formation of each of the components illustrated in the
flow chart in FIG. 16 can include multiple process steps. For example, in
the present example, forming the TFT electrodes (commonly referred to as
"pixel electrodes") on the first glass plate involves many different
process steps. Similarly, forming the color filters on the second glass
plate can involve numerous process steps. Typically, forming pixel
electrodes include multiple process steps to form the TFTs, ITO
electrodes, and various bus lines to the TFTs. In fact, forming the TFT
electrode layer is, in essence, forming a large integrated circuit and
involves many of the same deposition and photolithographic patterning
processing steps used in conventional integrated circuit manufacturing.
For example, various parts of the TFT electrode layer can be built by
first depositing a layer of material (e.g., a semiconductor, conductor,
or dielectric), forming a layer of photoresist over the layer of
material, exposing the photoresist to patterned radiation. The
photoresist layer is then developed, which results in a patterned layer
of the photoresist. Next, portions of the layer of material lying beneath
the patterned photoresist layer are removed in an etching process,
thereby transferring the pattern in the photoresist to the layer of
material. Finally, the residual photoresist is stripped from the
substrate, leaving behind the patterned layer of material. These process
steps can be repeated many times to lay down the different components of
the TFT electrode layer.

[0168] In general, the interferometry techniques disclosed herein can be
used to monitor overlay of different components of an LCD panel. For
example, during panel production, the interferometry techniques can be
used to determine overlay error between patterned resist layers and
features beneath the photoresist layer. Where measured overlay error is
outside a predetermined process window, the patterned photoresist can be
stripped from the substrate and a new patterned photoresist layer formed.